In the field, you can count branch "whorls" or clusters of branches on Douglas-fir, Sitka spruce, any of the pines, and a number of other conifers (but not western hemlock or western redcedar, among others). That will often get you within a year or two, depending on the age of the seedling stock planted in that harvest unit. If there is Douglas-fir, and there usually is, that is a good species to use for this approach. On poor sites, the branch whorls may be 12" or less apart, while on good sites, there may be 30" or more between whorls. That is the amount of vertical growth in a given year.
Hope that helps!